My online English teacher marked my answer for a practice question, the title, as wrong. And it revised that into sitting instead of sit. But could I just use that sentence, as it is, conveying the meaning that "she was seen in the act of sitting down"? I meant (sorry for my short English skill) she definitely started to sit, however her butt was not yet completely landed on the table, or just landed. Or, simply, does the verb Find not go well together with something in progress?
Your teacher is right.
To say what you are trying to convey, you could use one of these:
I saw her sit on the table.
I watched her sit on the table.
But found doesn't work; found means you saw someone in a certain state, not in a process.
Although you could say:
I found her in the act of sitting.
I probably would have corrected your sentence the same way your teacher did.